1 • Electrodialysis (ED). • Liquid membranes. • Pervaporation. Membrane Separations- There are other separation operations where a membrane is the responsible of the la selective separation of the compounds:• Dialysis.• Gas permeation (GP).• Electrodialysis (ED).• Liquid membranes.• Pervaporation.- In others, the membrane is not directly responsible for the separation but it actively participates:• Membrane extraction.• Membrane distillation.• Osmotic distillation.
2 Membrane SeparationsDead-endCross-flowType of filtration.
3 Simple scheme of a membrane module. Membrane SeparationsCA,f, CB,fCA,r, CB,rFeedRetentate(Concentrate)MembranePermeate(Filtrate)CA,p, CB,pSimple scheme of a membrane module.
4 Membrane Separations- Synthetic membranes are solid barriers that allow preferentially to pass specific compounds due to some driving force.+(Very) Simple scheme for some mechanisms of selective separation on a porous membrane.
5 • Pore size and structure Membrane Separations- The separation ability of a synthetic material depends on its physical, chemical properties.• Pore size and structure• Design• Chemical characteristics• Electrical charge
6 - Porous membranes give separation due to... Membrane Separations- The membranes can be roughly divided in two main groups: porous and non porous.- Porous membranes give separation due to...• size• shape• charge...of the species.- Non porous membranes give separation due to...• selective adsorption• diffusion...of the species.
7 Main parameters. - Rejection, R, if there is just one component (RO) Membrane SeparationsMain parameters.- Rejection, R, if there is just one component (RO)- Separation factor- Enrichment factorfor two or more component
8 Main parameters. - In RO, often we use the Recovery (Y) Membrane SeparationsMain parameters.- In RO, often we use the Recovery (Y)Qp: Permeate flowrate (m3/s)Qf: Feed flowrate (m3/s)
9 Membrane SeparationsMain parameters.- Passive transport in membranes. The permeate flux is proportional to a given driving force (some difference in a property).Driving forces: Pressure (total o partial) Concentration Electric Potential
10 Membrane processes and driving force. Membrane SeparationsMain parameters.Membrane processes and driving force.
11 Main parameters. - Permeate flux. Membrane SeparationsMain parameters.- Permeate flux.In MF and UF, porous membrane model is assumed, where the a stream freely flows through the pore. Then, the transport law follows the Hagen-Poiseuille equation.Jw: Solvent flux (m3/s·m2)Qw: Solvent flowrate (m3/s)Am: Membrane area (m2)r: Pore radius (m)d: Membrane thickness (m): Porosity: Viscosity (Pa ·s): TortuosityP: Hydraulic pressure difference (Pa)
12 Membrane SeparationsMain parameters.- The above model is good for cylindrical pores. However, if the membrane is rather formed by a aggregated particles, then the Kozeny-Carman relation works much better.JW: Solvent flux (m3/s·m2)QW: Solvent flowrate (m3/s)S: Particle surface area (m2/m3)K: Kozeny-Carman constantd: Membrane thickness (m)Am: Membrane area (m2): Viscosity (Pa ·s)
13 Formation of the polarisation layer. Membrane Separations- In the operations governed by the pressure, a phenomenon called concentration polarisation appears, which must be carefully controlled. This is due to the solute accumulation neighbouring the membrane surface.FeedPolarisation layermembranePermeateFormation of the polarisation layer.
14 - Concentration polarisation. Membrane Separations- Concentration polarisation.(It is not fouling!!!)
15 - Fouling: Irreversible reduction of the flux throughout the time. Membrane Separations- Fouling: Irreversible reduction of the flux throughout the time.• Pore size reduction by irreversible adsorption of compounds.• Pore plugging.• Formation of a gel layer over the membrane surface (cake).
16 • Structure: symmetric, asymmetric Membrane Separations- Membrane can be classified in several ways, but always there are arbitrary classifications.• Structure: symmetric, asymmetric• Configuration: flat, tubular, hollow fiber• Material: organic, inorganic• Surface charge: positive, negative, neutral• ...and even other divisions and subdivisions
17 - Integral: the layers are continuous. Membrane Separations- Structure:• Symmetric. Also called homogeneous. A cross section shows a uniform porous structure.• Asymmetric. In a cross section, one can see two different structures, a thin dense layer and below a porous support layer.- Integral: the layers are continuous.- Composites: the active layer (thickness μm) is supported over a highly porous layer ( μm), sometimes both layers are of different materials.
18 Membrane SeparationsSymmetric UF membrane of 0.45 m made of cellulose acetate (Millipore).
19 Membrane SeparationsSurfaceCross sectionSymmetric ceramic membrane of 0.2 m made of alumina (Al2O3) (AnoporeTM).
20 Asymmetric ceramic membrane made of -Al2O3 (Membralox). Membrane SeparationsAsymmetric ceramic membrane made of -Al2O3 (Membralox).